Embodiments relate generally to vibrators for marine geophysical surveys, and, more particularly, embodiments relate to the use of compliance chambers in marine vibrators to compensate for air-spring effects.
Sound sources are generally devices that generate acoustic energy. One use of sound sources is in marine seismic surveying. Sound sources may be employed to generate acoustic energy that travels downwardly through water and into subsurface rock. After interacting with the subsurface rock, for example, at the boundaries between different subsurface layers, some of the acoustic energy may be reflected back toward the water surface and detected by specialized sensors. The detected energy may be used to infer certain properties of the subsurface rock, such as the structure, mineral composition and fluid content. These inferences may provide information useful in the recovery of hydrocarbons.
Most of the sound sources employed today in marine seismic surveying are of the impulsive type, in which efforts are made to generate as much energy as possible during as short a time span as possible. The most commonly used of these impulsive-type sources are air guns that typically utilize compressed air to generate a sound wave. Other examples of impulsive-type sources include explosives and weight-drop impulse sources. Another type of sound source that can be used in marine seismic surveying includes marine vibrators, such as hydraulically powered sources, electro-mechanical vibrators, electrical marine seismic vibrators, and sources employing piezoelectric or magnetostrictive material. Marine vibrators typically generate vibrations through a range of frequencies in a pattern known as a “sweep” or “chirp.”
A marine vibrator may radiate sound by moving a number of sound radiating surfaces that are connected to a mechanical actuator. During this motion these surfaces displace a certain volume. This displaced volume may be the same outside and inside the marine vibrator. Inside the marine vibrator this volume displacement may cause a pressure variation that in absolute values increases substantially while the marine vibrator is lowered to increasing depths. As the internal gas (e.g., air) in the marine vibrator increases in pressure, the bulk modulus (or “stiffness”) of the internal gas also rises. Increasing the bulk modulus of the internal gas also increases the air-spring effect within the marine vibrator. As used herein, the term “air spring” is defined as an enclosed volume of air that may absorb shock or fluctuations of load due to the ability of the enclosed volume of air to resist compression. Increasing the stiffness of the air in the enclosed volume increases the air-spring effect and thus the ability of the enclosed volume of air to resist compression. This increase in the air-spring effect of the internal gas tends to be a function of the operating depth of the source. Further, the stiffness of the acoustic components of the marine vibrator and the internal gas are the primary determining factors in the marine vibrator's resonance frequency. Accordingly, the resonance frequency generated by the marine vibrator may undesirably increase when the marine vibrator is towed at depth, especially in marine vibrators where the interior volume of the marine vibrator may be pressure balanced with the external hydrostatic pressure. Hence, in applications it may be desirable that a resonance frequency can be retained independently of the operation depth and/or that the output resonance frequency can be controlled so as to be below and/or above its nominal resonance frequency.